INTRODUCTION. 



The present Part is devoted solely to the two Orders Ichthyopterygia 

 and Sauropterygia, which have taken so much space that it has 

 been deemed advisable to postpone the Chelonia till the following 

 Part, 



The whole of the species of both these Orders with which the 

 writer is acquainted, dating till the end of 1888, have been men- 

 tioned, although it has been found impossible to refer many of those 

 not represented in the Museum to their proper serial position. 



In revising the genera and species of these two orders the principle 

 adopted by Prof. Flower T in his arrangement of the Cetacea has 

 been followed ; species and genera not being recognized as such 

 unless capable of definition by more or less well-marked distinctive 

 characters. Hitherto, the opposite principle has been followed ; 

 and it is quite possible that in the present attempt to reduce the 

 number of ill-defined forms the writer may have carried this reduc- 

 tion too far, and that fuller knowledge will eventually show that in 

 some cases more than a single form has been included under one 

 specific heading. 



Generic terms, as a rule, have been employed in a wide sense. 

 Thus the genus Ichthyosaurus has been retained in the sense 

 in which it is used by previous writers as embracing by far the 

 greater number of the species of the order of which it is the type. 

 In this connection it may be mentioned that there was no real justifi- 

 cation for superseding the earlier name Proteosaurus by the later 

 Ichthyosaurus ; but since the latter name has been universally 

 adopted, the writer, after consultation with the Director of the 



1 See Preface to 'List of Specimens of Cetacea in the British Museum ' 

 (1885). 



